1. Technical Field of the Invention
The present invention relates to alloys of the Al—Zn—Mg type with good mechanical strength, and more particularly alloys intended for welded constructions such as the structures employed in the field of shipbuilding, motor vehicle bodywork, industrial vehicles and fixed or mobile tanks.
2. Prior Art Description of Related Art
To manufacture welded structures, aluminium alloys of the 5xxx series (5056, 5083, 5383, 5086, 5186, 5182, 5054 etc.) and 6xxx series (6082, 6005A etc.) are generally used. 7xxx alloys with a low copper content, that are weldable (such as 7020, 7108 etc.), are also adapted for making welded parts in so far as they have very good mechanical properties, even after welding. These alloys are however subject to problems of layer corrosion (in the T4 state and in the weld affected zone) and stress corrosion (in the T6 state).
Alloys of the 5xxx group (Al—Mg) are usually used in the H1x (strain-hardened), H2x (strain-hardened then restored), H3x (strain-hardened and stabilised) or O (annealed) states. The choice of temper depends on the compromise between mechanical strength, corrosion strength and formability that is targeted for a given use.
7xxx alloys (Al—Zn—Mg) are known as having “structural hardening”, which means that they acquire their mechanical properties through precipitation of the alloying elements (Zn, Mg). The man skilled in the art knows that, to obtain these mechanical properties, hot transformation by rolling or extrusion is followed by solution treatment, quenching or an ageing treatment. The purpose of these operations, which are carried out in most cases separately, is respectively to dissolve the alloying elements, to keep them in a supersaturated solid solution form at ambient temperature, and lastly to precipitate them in a controlled manner.
Alloys of the 6xxx (Al—Mg—Si) and 7xxx (Al—Zn—Mg) groups are usually used in the age treated state. In the case of products in the form of sheets or strips, the ageing treatment giving the greatest mechanical strength is denoted T6, when forming by rolling or extrusion is followed by a separate solution treatment and quenching.
When dimensioning a structure, the parameters governing user choice are essentially the static mechanical characteristics, in other words, the fracture strength Rm, the yield strength Rp0.2, and the elongation at fracture A. Other parameters coming into play, depending on the specific needs of the targeted application, are the mechanical characteristics of the welded joint, the corrosion (layer and stress) strength of the sheet and welded joint, the fatigue strength of the sheet and welded joint, the crack propagation strength, the fracture toughness, the dimensional stability after cutting or welding, and resistance to abrasion. For each targeted use, an adapted compromise needs to be found between these different properties.
The possibility of producing laminated products of constant quality on an industrial basis with a manufacturing process that is as straightforward as possible and a production cost as low as possible is also an important factor in the choice of material.
For 7xxx alloys (Al—Zn—Mg), the prior art offers a number of ways to improve the compromise of properties.
The patent GB 1 419 491 (British Aluminium) discloses a weldable alloy containing 3.5-5.5% zinc, 0.7-3.0% magnesium, 0.05-0.30% zirconium, optionally up to 0.05% each of chrome and manganese, up to 0.10% iron, up to 0.075% silicon, and up to 0.25% copper.
The article “New weldable AlZnMg alloys” by B. J. Young, which appeared in Light Metals Industry, November 1963, mentions two compound alloys:
Zn 5.0% Mg 1.25% Mn 0.5% Cr 0.15% Cu 0.4% and
Zn 4.5% Mg 1.2% Mn 0.3% Cr 0.2%.
The article mentions the use of this type of alloy for lorry skips and in shipbuilding.
The patent FR 1 501 662 (Vereinigte Aluminium-Werke Aktiengesellschaft) describes a weldable compound alloy                Zn 5.78% Mg 1.62% Mn 0.24% Cr 0.13% Cu 0.02% Zr 0.17%used in the form of 4 mm thick sheets, after solution treatment for an hour at 480° C., quenching in water and a two stage ageing treatment (24 hours at 120° C., then 2 hours at 180° C.), to manufacture armour plating.        
The patent U.S. Pat. No. 5,061,327 (Aluminum Company of America) describes a process of manufacturing a laminated product in an aluminium alloy comprising the casting of a plate, homogenising, hot rolling, reheating the stock to a temperature between 260° C. and 582° C., fast-cooling it, a precipitation treatment at a temperature between 93° C. and 288° C., then cold or hot rolling at a temperature not exceeding 288° C.
The problem to which the present invention tries to respond is first of all to improve the compromise of certain properties of Al—Zn—Mg alloys in the form of sheets or strips, namely the compromise between the mechanical characteristics (determined on the base metal and on the welded joint), and the corrosion strength (layer corrosion and stress corrosion). Furthermore, the aim is to make these products using a production process that is as straightforward and reliable as possible, allowing them to be manufactured with a manufacturing cost that is as low as possible.